1. Field of the Invention
The present invention is directed to an improved process for the preparation of (cyclo)aliphatic polyisocyanates containing uretdione and isocyanurate groups from (cyclo)aliphatic diisocyanates with the aid of tertiary phosphine catalysts, to the modified polyisocyanates obtained by this process and to their use as the isocyanate component in two-component polyurethane coatings.
2. Description of the Prior Art
The preparation of lacquer polyisocyanates containing uretdione and isocyanurate groups by the oligomerization of monomeric starting diisocyanates with the aid of organic phosphines as catalysts followed by termination of the modifying reaction at the desired degree of oligomerization is known (see e.g. DE-OS 1,670,667, DE-OS 1,670,720, DE-OS 1,954,093 or US-PS 4,614,785). The polyisocyanates containing uretdione and isocyanurate groups obtained by the processes described in these prior publications generally have an iodine color number of at least 5, which limits their usefulness for light colored polyurethane lacquers.
Therefore, it is an object of the present invention to improve the processes disclosed in these prior publications for the preparation of polyisocyanates containing uretdione and isocyanurate groups from (cyclo)aliphatic starting diisocyanates so that the products obtained have iodine color numbers of at most 3, preferably 0 to 1, or Hazen color numbers according to DIN 53 409 of not more than 250, preferably below 100 .
This object may be achieved by the process according to the invention described below.